With the use of the Boltzmann superposition principle, a statistical theory of rubber elasticity, and phenomenological theory, the hysteresis loss at high strain (greater than 100%) of nonlinear viscoelastic materials has been quantified in terms of material parameters. The theory incorporates both experimental and analytical parameters. The experimental results for natural rubber (NR) and styrene-butadiene rubber (SBR) vulcanizates having variations of loading of carbon black, silica, clay, resin and curatives are in good accord with the theoretical predictions.